Exploiting pi-acceptors for the determination of thyroid hormones (T3 and T4) using a single interface flow system

A fully automated methodology was developed for the determination of the thyroid hormones levothyroxine (T4) and liothyronine (T3). The proposed method exploits the formation of highly coloured charge-transfer (CT) complexes between these compounds, acting as electron donors, and pi-acceptors such as chloranilic acid (CIA) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). For automation of the analytical procedure a simple, fast and versatile single interface flow system (SIFA)was implemented guaranteeing a simplified performance optimisation, low maintenance and a cost-effective operation. Moreover, the single reaction interface assured a convenient and straightforward approach for implementing job`s method of continuous variations used to establish the stoichiometry of the formed CT complexes. Linear calibration plots for levothyroxine and liothyronine concentrations ranging from 5.0 x 10(-5) to 2.5 x 10(-4) mol L(-1) and 1.0 x 10(-5) to 1.0 x 10(-4) mol L(-1), respectively, were obtained, with good precision (R.S.D. <4.6% and <3.9%) and with a determination frequency of 26 h(-1) for both drugs. The results obtained for pharmaceutical formulations were statistically comparable to the declared hormone amount with relative deviations lower than 2.1%. The accuracy was confirmed by carrying out recovery studies, which furnished recovery values ranging from 96.3% to 103.7% for levothyroxine and 100.1% for liothyronine. (C) 2009 Elsevier B.V. All rights reserved.